It's a chemical composition of the first and third most abundant elements in the universe! Water is readily made as a side product of almost every exothermic reaction which means it's a certainty that if aliens needed water it would be a damn sight easier to synthesize it than it would be to travel across light-years of space to steal it from another life form!

I thought someone was going to say "resources" as though there are not thousands upon thousands of planets, planetoids and asteroids containing minerals and ores that are considerably closer than earth. But no, you said something even stupider!

I've heard the big game theory but while there is definitely a precedent for people to travel large distances to hunt large dangerous animals, they don't tend to hunt them using their entire war machine. Hunters want to take home trophies, not conquer the land of the bears.